Cops and robbers MTA servers are Multi Theft Auto servers where a players can be Cops or Robbers. After joining Cops and Robbers you can pick your role and then run with it, on some MTA servers you can also be civilian.
Anyone can be Cop?
This depends on MTA server, some Cops or Robbers servers would allow anyone to pick any role, while other servers would require players to apply for some roles to prevent abuse.